With the increasing popularity of portable electronic appliances, there have been extensive efforts to reduce the size and weight of their various components. In many circumstances, one of the principle limitations to such size reductions is the energy storage device. Accordingly, there have been ongoing efforts to develop energy storage devices that are lighter in weight, smaller in size and capable of storing the maximum electrical charge per unit weight and/or size. In order to remain economical during extended uses, it is important that the batteries be rechargeable.
In recent years there have been substantial improvements in rechargeable battery technology; however, existing battery designs remain quite heavy in comparison with other electrical components. There are numerous batteries on the market, with the different designs using a wide variety of chemistries. The two most popular types of batteries that are used to power portable electronic devices are nickel cadmium (NICAD) batteries and sealed lead acid batteries.
The most common and probably the best known battery construction is lead acid. Substantially all the existing automotive battery designs are lead acid based. One advantage of lead acid batteries is that they have very repeatable power delivery characteristics and may be recharged and overcharged repeatedly with minimal damage to the cells. Additionally, the power curve is consistent enough that the charge remaining in a cell at any given time can be relatively accurately predicted by merely measuring the cell's potential. Thus, a user can be easily warned well in advance of a loss of power. The major drawback of lead acid batteries is that they tend to be heavy; traditionally, large lead grids are used to form the battery's plates and the cells are flooded with an acid based electrolyte. Typically, the grids are structurally self supporting which increases their weight. Additionally, in order to ensure a reliable electrolytic seal between adjacent cells, in practice it is generally necessary to fomm a gas tight seal about each cell.
A significant improvement to the traditional lead acid battery design is the recombinant battery. The recombinant lead acid battery differs from its predecessors in that substantially all of the electrolyte is absorbed within the separator between adjacent plates and/or an active paste applied directly to the plates. Gases evolved during operation or charging are not normally vented into the atmosphere, but rather are induced to recombine within the battery. With such an arrangement, no free acid is available, which allows the battery to be sealed and maintenance free. The elimination of free acid also provides a safer battery design.
Several refinements have been made to recombinant lead acid batteries. For example, U.S. Pat. No. 3,862,861 describes a significantly improved lead acid battery design, which uses flexible, non-self-supporting grids within a recombinant lead acid battery design. Specifically, the flexible grids are fabricated from a very high purity lead and are separated only by a microporous fiberglass material that retains the electrolyte within the separator itself. Such an arrangement improves the energy storage per unit weight characteristics of lead acid batteries since the separator structure is significantly lightened.
U.S. Pat. Nos. 4,383,011, 4,525,438 and 4,659,636 all describe alternative recombinant lead acid battery designs. The '636 design stacks a plurality of flattened cells in order to produce a somewhat flattened battery unit. Unfortunately, even these relatively improved lead acid battery designs only match the energy density (watt-hours of energy stored per unit weight of battery) and the packaging density (watt-hours of energy stored per unit volume of battery) of NICAD cells.
An important technical requirement of lead acid batteries is that the spacing between plates must be maintained at a constant distance. As is well known in the art, if the plates of a lead acid battery are not sufficiently constrained, the plates will expand and the battery degrades relatively quickly. Accordingly, the casing must be sufficiently strong to prevent separation of the plates under the influence of the considerable forces that can act on the plates during a charge/discharge cycle.
Two areas of prior art recombinant batteries are particularly bulky and/or heavy. The first is the electrodes which are typically fabricated from lead grids and the second is the casing structure which is typically heavily reinforced. Prior art batteries, such as those described above, merely increase the thickness of the casing in order to prevent deformation of their casings and/or electrodes. However, such external support is disadvantageous since it is quite bulky. Therefore, there is a need for an improved light weight battery plate support structure.
Although nickel-cadmium batteries tend to have slightly better energy and packaging density characteristics than conventional lead acid batteries, they also have numerous drawbacks for powering portable electronic units. Among the most noticeable is that their power delivery curves vary a great deal depending upon their charging and recharging history. Thus, they are unsuitable for use in devices that must be recharged at varying intervals. Accordingly, there is a need for an improved battery design that has improved energy density and packaging density characteristics.